Seastar threat grows

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A marine invader that has 10 testes and will eat anything from sea squirts to dead human bodies is expanding its range in Australia - threatening to destroy commercial shellfish industries and wreaking environmental havoc.

The northern Pacific seastar looks pretty, but its habit of hitching rides around the world in the ballast water of ships and then growing to plague proportions in new habitats is making it one of Australia's ugliest marine creatures.

Densities of the seastar in the Derwent estuary in Hobart have been as high as seven per square metre - among the highest in the world - and now a new report shows a separate invasion in Victoria.

Recent estimates suggest the population in Port Phillip Bay has reached 12 million in the two years since they were first spotted, despite attempts to destroy the pests.

As the creatures have not been found between Hobart and Port Phillip Bay, it's thought the expansion of their range is not natural but rather a result of ship travel between the two ports.

The stars are easily sucked up in ballast water and can grow on boat hulls. They can also travel in oyster seed trays and mussel ropes, which are often moved between areas to maximise shellfish growth.

Scientists believe infection with a natural parasite or disease is probably the only realistic long-term means of control.

In Japan, where seastars are native, a "castrating" parasite had raised hopes it could be used as a biological control. The tiny parasites invade the stars' testes and devour their sperm. But because seastars have 10 testes and the parasite does not usually invade all of them, a search for a more aggressive parasite is under way.

A special feature, based on the diary of Dr Louise Goggin's recent trip to Korea to search for other potential biological controls, will be published on The Lab on June 18.